Optic lobe development.

The optic lobes comprise approximately half of the fly's brain. In four major synaptic ganglia, or neuropils, the visual input from the compound eyes is received and processed for higher order visual functions like motion detection and color vision. A common characteristic of vertebrate and invertebrate visual systems is the point-to-point mapping of the visual world to synaptic layers in the brain, referred to as visuotopy. Vision requires the parallel extraction of numerous parameters in a visuotopic manner. Consequently, the optic neuropils are arranged in columns and perpendicularly oriented synaptic layers that allow for the selective establishment of synapses between columnar neurons. How this exquisite synaptic specificity is established during approximately 100 hours of brain development is still poorly understood. However, the optic lobe contains one of the best characterized brain structures in any organism-both anatomically and developmentally. Moreover, numerous molecules and their function illuminate some of the basic mechanisms involved in brain wiring. The emerging picture is that the development of the visual system of Drosophila is (epi-)genetically hard-wired; it supplies the emerging fly with vision without requiring neuronal activity for fine tuning of neuronal connectivity. Elucidating the genetic and cellular principles by which gene activity directs the assembly of the optic lobe is therefore a fascinating task and the focus of this chapter.